multi client can not upload file at same time in node.js

am using express.js and uploadify to upload large file to node server, everything works fine except when more than one user login and try to upload file at same time, well it still works but it seems that the server can only upload one file at a time, so the user have to wait until the other user finish their uploading, this is so unacceptable.

here is server side code

exports.upload = function(req, res,next){
    // console.log( req.body);
    // console.log(req.files);
    var tmp_path = req.files.product_video.path;
    var target_path = 'F:/shopping/shop/' +req.body.shop_id+'/'+ req.files.product_video.name;

    fs.rename(tmp_path, target_path, function(err) {
        if (err) {
            console.log(err)
        }
        else{
            fs.unlink(tmp_path, function() {
                if (err){
                    console.log(err)
                }else{
                    exec("C:/ffmpeg/bin/ffmpeg -i shop/"+ req.body.shop_id+ '/' + req.files.product_video.name  + " -ss 00:01:00.00 -r 1 -an -vframes 1 -s 250x150 -f mjpeg shop/"+ req.body.shop_id+ '/'  + req.files.product_video.name  + "_thumbnail.jpg", function(err){

                        var data = {
                            'thum_src':'shop/'+ req.body.shop_id+ '/'  + req.files.product_video.name  + "_thumbnail.jpg",
                            'video_name':req.files.product_video.name,
                        }
                        res.send(data);
                    });
                }
            });
        }

    });
};

here is front end code

 $('#input_product_video').uploadify({
        'formData':{'shop_id':$('#shop_id').val()},
        'buttonText'    : 'add',
        'fileSizeLimit' : '100MB',
        'fileObjName' : 'product_video',
        'uploader'    : '/uploads',
        'swf'         :'/public/javascripts/lib/uploadify/uploadify.swf',
        'onUploadSuccess':function(file,data){
             console.log(file);
             console.log(JSON.parse(data));
             console.log(response);

        }
    });